Add support for host-installed SCI handlers.

This change adds support to allow hosts to install System Control
Interrupt handlers. Certain ACPI functionality requires the host
to handle raw SCIs. For example, the "SCI Doorbell" that is defined
for memory power state support requires the host device driver to
handle SCIs to examine if the doorbell has been activated. Multiple
SCI handlers can be installed to allow for future expansion.
Debugger support is included.
Lv Zheng, Bob Moore. ACPICA BZ 1032.
AcpiExec: Add support/test for SCI handlers.

Adds some testing/exercise of the SCI handler installation and
removal code.